The present invention relates to an optical blood detector system that rapidly detects the presence of blood due to a blood leak in a system. The blood detector system contains a reusable blood sensor that is able to accurately detect the presence of blood in, for example, an extracorporeal blood treatment system by optically sensing light from a sensing region and determining if the light came from a leaked blood. The blood sensor may be responsive to reflected light or light emitted from blood due to bio-fluorescence excited by a light source in the blood detector system. The blood detector system can be placed against absorbent material adjacent to an intravenous needle injection site and quickly detect any blood wicked into the absorbent material. The blood detector system can eliminate the need for medical personnel to continuously inspect numerous patients visually for potentially fatal blood leaks due to needle dislodgement.